STOP ROP was a masked trial designed to determine if moderate elevations of inspired oxygen in premature infants with prethreshold retinopathy of prematurity (ROP) would decrease the progression of the disease, thereby decreasing the risk of blindness from ROP-induced retinal detachment. Infants with prethreshold ROP were randomized into two SpO2 groups: "conventional" SpO2 (89-94%) and "supplemental" SpO2 (95-99%) and remained there for at least two weeks. To meet entry criteria for STOP-ROP, an infant had to be judged by two examiners, one of whom was a study certified ophthalmologist, to have prethreshold ROP in one or both eyes. The primary outcome was the proportion of infants in each group with one or both eyes progressing to threshold ROP.